Process for coating electrically conducting substrates, substrates coated by this process and aqueous electrocoating baths

ABSTRACT

The invention relates to a cathodic electrocoating process, which uses electrocoating baths which contain a cationc amine-modified epoxy resin otainable by reacting 
     (A) a diepoxy compound of an epoxy equivalent weight below 2000 
     B) a compound monofunctionally reactive toward the epoxide groups and containign an alcoholic OH group, a phenolic OH group or an SH group, and 
     (C) an amine, 
     the components (A) and (B) being used in molar ratio of 10:1 to 1:1, preferably 4:1 to 1.5:1 and the positive charges being introduced by protonization of the reaction product and/or by use of amine salts as the component (C). The electrocoating baths used are distinguished by containing at least 7.5% by weight of a polyoxyalkylenepolyamine, the percentage by weight referring to the total amount of binders contained in the electrocoating bath.

This is a continuation of application Ser. No. 07/548,999, filed Sep. 181990, now abandoned.

The invention relates to a process for coating electric ally conductingsubstrates, in which process

(1) the substrate is immersed in an aqueous electrocoating bath whichcontains a cationic amine-modified epoxy resin obtainable by reacting

(A) a diepoxy compound of an epoxide equivalent weight below 2000

(B) a compound monofunctionally reactive toward the epoxide groups andcontaining an alcoholic OH group, a phenolic OH group or an SH group,and

(C) an amine,

the components (A) and (B) being used in a molar ratio of 10:1 to 1:1,preferably 4:1 to 1.5:1 and the positive charges being introduced byprotonization of the reaction product and/or by use of amine salts asthe component (C);

(2) the substrate is connected as cathode,

(3) a film is deposited on the substrate by the action of directcurrent,

(4) the substrate is removed from the electrocoating bath and

(5) the deposited paint film is baked.

The invention also relates to substrates coated by the process accordingto the invention and to the electrocoating baths employed in the processaccording to the invention.

The cathodic electrocoating process described above is a paintingprocess frequently used primarily for priming, in particular for primingautomotive bodies.

A process according to the preamble of patent claim 1 is known fromDE-OS 3,518,732, DE-OS 3,409,188 and DE-OS 3,518,770.

Surface defects (particularly craters), well-known to a person skilledin the art, disadvantageously occur in the processes described in DE-OS3,409,188, DE-OS 3,518,732. and DE-OS 3,518,770.

The object forming the basis of the present invention is to makeavailable a novel process according to the preamble of patent claim 1,using which it is possible to produce paint films which, compared withpaint films of prior art, exhibit fewer and/or more faintly pronouncedsurface defects and do not give rise to defects due to adhesion failurein overcoated paint films.

The object is achieved according to the invention by a process accordingto the preamble of patent claim 1, wherein the electrocoating bathcontains at least 7.5% by weight of a polyoxyalkylenepolyamine or amixture consisting of several polyoxyalkylenepolyamines of differentchemical structures, the percentage by weight referring to the totalamount of binders contained in the electrocoating bath.

The advantages achieved by the invention are essentially to be found inthe fact that it is possible, using the process according to theinvention, to obtain paint films which, compared with paint films ofprior art, exhibit fewer and/or more faintly pronounced surface defectsand do not give rise to defects due to adhesion failure in overcoatedpaint films, especially in overcoated paint films containing alkydresins as binders.

Further important advantages achieved by the process according to theinvention lie in the fact that, compared with prior art, the paint filmsobtained by the process according to the invention possess greater filmthicknesses and greater flexibility.

U.S. Pat. No. 3,975,250 discloses cationic reactive plasticizers,suitable for use in electrocoating baths, which are prepared by thereaction of partly blocked polyisocyanates withpolyoxypropylenediamines. However, an average person skilled in the art,faced with the object forming the basis of the present invention, isunable to infer from U.S. Pat. No. 3,975,250 any information forachieving this object.

U.S. Pat. No. 4,423,166 discloses an electrocoating process, wherein theelectrocoating bath contains as anticratering agent an aduct ofpolyoxyalkylenepolyamine and a polyepoxide. However, the processdisclosed in U.S. Pat. No. 4,423,166 does not use electrocoating bathsaccording to the preamble of patent claim 1.

If, however, the anticratering agents disclosed in U.S. Pat. No.4,423,166 are employed in a process according to the preamble of patentclaim 1, then it is true that the tendency for surface defect formationis reduced, but instead problems due to adhesion failure occur in theovercoated paint films. Particularly serious problems occur inovercoated paint films containing alkyd resins as binders.

The advantages achieved by the present invention are the moresurprising, since electrocoating baths are described in U.S. Pat. No.4,423,166, column 18, which contain up to 7.0% by weight of apolyoxyalkylenepolyamine based on the total amount of binders in theelectrocoating bath, but produce paint films with pronounced surfacedefects (cf. Table 1 in U.S. Pat. No. 4,423,166).

Electrocoating baths for cathodic electrocoating are preferablypreferred by first preparing an aqueous dispersion which contains acationic resin and, if appropriate, a crosslinking agent and othercustomary additives, such as, for example, antifoams etc.

A pigment paste is then incorporated in this aqueous dispersion.

The pigment paste consists of a ground resin and pigments and/orfillers. In addition, the pigment paste may also contain otheradditives, such as, for example, plasticizers wetting agents,antioxidants etc.

Examples of pigments and fillers which the pigment paste may contain,are: titanium dioxide, antimony oxide, zinc oxide, basic lead carbonate,basic lead sulfate, barium carbonate, porcelain, clay, calciumcarbonate, aluminum silicate, silicon dioxide, magnesium carbonate,magnesium silicate, cadmium yellow, cadmium red, carbon black,phthalocyanin blue, chrome yellow, toluidyl red and hydrated iron oxide.

The preparation of the pigment paste is generally known and need not beexplained here in greater detail (cf., for example, D. H. Parker,Principles of Surface Coating Technology, Interscience Publishers, NewYork (1965); R. L. Yates, Electropainting, Robert Draper Ltd.,Teddington/England (1966); H. F. Payne, Organic Coating Technology,volume 2, Wiley and Sons, New York (1961).

The pigment paste is added to the aqueous dispersion described above insuch an amount that the finished electrocoating bath possesses thecharacteristics required for the deposition. In most cases the weightratio of pigments or filler to the total amount of cationic resincontained in the electrocoating bath is 0.05 to 0.5.

After the aqueous dispersion and the pigment paste have been combinedand the solids content of the combination has been suitably adjusted, anelectrocoating bath ready-for-use is obtained.

The electrocoating baths used according to the invention contain acationic amine-modified epoxy resin obtainable by reacting

(A) a diepoxy compound of an epoxide equivalent weight: below 2000

(B) a compound monofunctionally reactive toward the epoxide groups andcontaining an alcoholic OH group, a phenolic OH group or an SH group,and

(C) an amine,

the components (A) and (B) being used in a molar ratio of 10:1 to 1:1,preferably 4:1 to 1.5:1, and the positive charges being introduced byprotonization of the reaction product and/or by use of amine salts asthe component (C).

Cationic amine-modified epoxy resins of the type described above aredisclosed in DE-OS 3,409,188, DE-OS 3,518,732 and DE-OS 3,518,770.

Any compound containing two reactive epoxide groups and having anepoxide equivalent weight below 2000, preferably below 1000,particularly preferably below 500, may in principle be used as thecomponent (A).

Examples of diepoxide compounds which may be employed as the component(A), are diglycidyl ethers of polyphenols, diglycidyl ethers ofdialcohols and diglycidyl esters of dicarboxylic acids.

Diglycidyl ethers of polyphenols, in particular diglycidyl ethers ofbisphenol A, are particularly preferably used as the component (A).Diglycidyl ethers of polyphenols may be obtained by the reaction ofepihalohydrins with polyphenols.

Mixtures of various diepoxide compounds may of course also be employedas the component (A).

Any compound monofunctionally reactive toward epoxide groups, whichcontains an alcoholic OH group reactive toward epoxide groups, aphenolic OH group reactive toward epoxide groups or an SH group reactivetoward epoxide groups in the molecule, may in principle be used as thecomponent (B).

Compounds which contain a phenolic OH group reactive toward epoxidegroups in the molecule, in particular alkyl phenols, are particularlypreferred as the component (B).

Mixtures of various compounds monofunctionally reactive toward epoxidegroups, may of course also be used.

Primary or secondary amines or their salts, salts of tertiary amines ormixtures of these compounds may be used as the component (C).

Water-soluble amines are preferably used as the component (C). Examplesof suitable amines are monoalkylamines and dialkylamines, such asmethylamine, ethylamine, propylamine, butylamine, dimethylamine,diethylamine, dipropylamine, methylbutylamine etc. Alkanolamines, suchas, for example, methylethanolamine and diethanolamine, may also be usedas the components (C). Ketimines of polyamines with primary andsecondary amino groups may be used as the component (C). In addition,dialkylaminoalkylamines, such as, for example, dimethylaminoethylamine,diethylaminopropylamine and dimethylaminopropylamine are also suitableas the component (C).

Low-molecular amines are employed in most cases as the component (C).However, it is also possible to use high-molecular monoamines.

In many cases several different amines are used as the component (C).

The cationic amine-modified epoxy resin under discussion may be producedby preparing from the components (A) and (B) in a first step anintermediate containing epoxide groups having a molecular weight of 400to 5000. This reaction is preferably carried out in an inert organicsolvent at reaction temperatures from 100° to 190° C. in the presence ofsuitable catalysts (for example tertiary amines) and is controlled bydeterminations of the epoxide equivalent weights.

The epoxide groups of the intermediate obtained in this manner are thenreacted at least in part with the component (C). In those cases where noamine salts are used as the-component (C), this reaction is likewisecarried out preferably in an organic solvent. The reaction betweenamines and compounds containing epoxide groups frequently sets inalready when the two coreactants are mixed. Depending on the desiredcourse of reaction, particularly for the reaction to go to completion,it is advisable to raise the reaction temperature to 50°-150° C. Whenamine salts are used as the component (C), then it is preferable tocarry out the corresponding reaction in an aqueous reaction medium.

It is frequently desirable chemically to modify the binder stillfurther. For this purpose at least a part of the reactive groups of theintermediate obtained from the components (A) and (B) or of the reactionproduct obtained from the components (A), (B) and (C) is reacted withmodifying compounds.

Examples of modifying compounds are:

a) Compounds containing carboxyl groups, such as saturated orunsaturated monocarboxylic acids (for example benzoic acid, linseed oilfatty acids, 2-ethylhexanoic acid, versatic acid), aliphatic,cycloaliphatic and/or aromatic dicarboxylic acids of various chainlengths (for example adipic acid, sebacic acid, isophthalic acid ordimeric fatty acids), hydroxyalkylcarboxylic acids (for example lacticacid, dimethylolpropionic acid) as well as esters containing carboxylgroups, or

b) compounds containing amino groups, such as diethylamine orethylhexylamine or diamines with secondary amino groups, for exampleN,N'-dialkylalkylenediamines, such as dimethylethylenediamine,N,N'-dialkylpoloxyalkyleneamines, such asN,N'-dimethylpolyoxypropylenediamine, cyanoalkylated alkylenediamines,such as bis-N,N'-cyanoethylethylenediamine, cyanoalkylatedpolyoxyalkyleneamines, such as bis-N,N'-cyanoethylpolyoxypropylenediamine, polyaminoamides, such as, for example, versamides,particularly reaction products containing terminal amino groups,obtained from diamines (for example hexamethylenediamine),polycarboxylic acids, particularly dimeric fatty acids andmonocarboxylic acids, particularly fatty acids, or the product obtainedby reacting one mol of diaminohexane with two mol of monoglycidyl ethersor monoglycidyl esters, particularly glycidyl esters of α-branched fattyacids, such as versatic acid, or

c) compounds containing hydroxyl groups, such as neopentyl glycol,bis-ethoxylated neopentyl glycol, neopentyl glycol hydroxypivalate,dimethylhydantoin-N,N'-diethanol, hexane-1,6-diol, hexane-2,5-diol,1,4-bis(hydroxymethyl)cyclohexane,1,1-isopropylidene-bis-(p-phenoxy)-2-propanol, trimethylolpropane,pentaerythritol or aminoalcohols, such as triethanolamine,methyldiethanolamine or alkylketimines containing hydroxyl groups, suchas aminomethylpropanediol-1,3-methylisobutylketimine ortris(hydroxymethyl)aminomethanecyclohexanoneketimine as well aspolyglycol ethers, polyester polyols, polyether polyols,polycaprolactone polyols, polycaprolactam polyols of variousfunctionality and molecular weights, or

d) saturated or unsaturated fatty acid methyl esters which in thepresence of sodium methylate are esterified by the hydroxyl groups ofthe epoxy resins.

The positive charges required for the water thinnability and thecapability to be electrically deposited may be introduced in the bindermolecule by protonization with water-soluble acids (for example boricacid, formic acid, lactic acid, preferably acetic acid) and/or by theuse of amine salts as the components (C).

The cationic amine-modified epoxy resins used according to the inventionare essentially free from epoxide groups, i.e. their epoxide groupcontent is so low that no cross-linking reactions via the epoxide groupscan occur either prior to or after the deposition of the paint films.The cationic amine-modified epoxy resins used according to the inventionpreferably contain no free epoxide groups.

The cationic amine-modified epoxy resin used in the electrocoating bathsaccording to the invention may be converted by chemical modification toa self-crosslinking cationic resin. A self-crosslinking cationic resinmay be obtained by reacting the cationic amine-modified resin or aprecursor of the cationic amine-modified epoxy resin with partly blockedpolyisocyanate containing on average one free isocyanate group permolecule. However, the electrocoating baths preferably used in theprocess according to the invention are those which contain the cationicamine-modified epoxy resin under dicussion as a non-self-crosslinkingcationic resin in combination with a suitable crosslinking agent.

Examples of suitable crosslinking agents are phenoplasts, polyfunctionalMannich bases, melamine resins, benzoguanamine resins, blockedpolyisocyanates and compounds containing at least two groups of thegeneral formula R¹ --O--CO--.

The radical R¹ denotes R² --O--CO--CH₂ --, R³ --CHOH--CH₂ -- or R⁴--CHOR⁵ --CHOH--CH₂ --

R² denotes alkyl

R³ denotes H, alkyl, R⁶ --O--CH₂ -- or R⁶ --CO--O--CH₂ --

R⁴ denotes H or alkyl

R⁵ denotes H, alkyl or aryl

R⁶ denotes alkyl, cycloalkyl or aryl

Examples of compounds containing at least two groups of the generalformula R¹ --O--CO--, are bis(carbalkoxymethyl) azelate,bis(carbalkoxymethyl) sebacate, bis(carbalkoxymethyl) adipate,bis(carbalkoxymethyl) decanoate, bis(carbalkoxymethyl) terephthalate,bis(2-hydroxybutyl) azcelate and bis(2-hydroxyethyl) terephthalate.

The electrocoating baths particularly preferably used in the processaccording to the invention are those which contain the cationicamine-modified epoxy resin under discussion as a non-self-crosslinkingcationic resin in combination with a blocked polyisocyanate ascrosslinking agent.

Any polyisocyanate in which the isocyanate groups have been reacted witha compound in such a manner that the blocked polyisocyanate formed isnon-reactive toward hydroxyl and amino groups at room temperature, butbecomes reactive at elevated temperatures, usually in the range fromabout 90° C. to about 300° C., may be used as the blockedpolyisocyanate. Any organic polyisocyanate suitable for the crosslinkingmay be employed in the preparation of the blocked polyisocyanates.Isocyanates containing about 3 to 36, particularly about 8 to about 15carbon atoms, are preferred. Examples of suitable diisocyanates arehexamethylene diisocyanate, 2,4-toluylene diisocyanate, 2,6-toluylenediisocyanate and1-isocyanatomethyl-5-isocyanato-1,3,3-trimethylcyclohexane.Polyisocyanates of higher isocyanate functionality may also be used.Corresponding examples are trimerized hexamethylene diisocyanate andtrimerized isophorone diisocyanate. In addition, mixtures ofpolyisocyanates may be also used. The organic polyisocyanates which aresuitable crosslinking agents in the invention, may also be prepolymersderived, for example, from a polyol including a polyether polyol or apolyester polyol.

Any suitable aliphatic, cycloaliphatic or aromatic alkylmonoalcohol maybe used for the blocking of the polyisocyanates. Examples are aliphaticalcohols such as methyl, ethyl, chloroethyl, propyl, butyl, amyl, hexyl,heptyl, octyl, nonyl, 3,3,5-trimethylhexyl, decyl and lauryl alcohol,cycloaliphatic alcohols such as cyclopentanol and cyclohexanol, aromaticalkylalcohols such as phenylcarbinol and methylphenylcarbinol.

Other suitable blocking agents are hydroxylamines such as ethanolamine,oximes such as methyl ethyl ketonoxime, acetonoxime andcyclohexanonoxime, or amines such as dibutylamine and diisopropylamine.These polyisocyanates and blocking agents may be also used in suitableproportions for the preparation of the partly blocked polyisocyanatesreferred to above.

The crosslinking agent is usually added in an amount of 5 to 60% byweight, preferably 20 to 40% by weight, based on the total amount ofcrosslinkable cationic resin contained in the electrocoating bath.

The proportion of the cationic amine-modified epoxy resin described inthe preamble of patent claim 1, of the total amount of binders containedin the electrocoating bath is preferably 90 to 40, particularlypreferably 75 to 60% by weight in those cases in which the cationicamine-modified epoxy resin is present as a non-self-crosslinking resin,and is preferably 100 to 70% by weight in those cases in which thecationic amine-modified epoxy resin is present as a self-crosslinkingresin.

It is an essential part of the invention that the electrocoating bathsused in the process according to the invention contain at least 7.5 % byweight of a polyoxyalkytene polyamine, the percentage by weightreferring to the total amount of binders contained in the electrocoatingbath.

Polyoxyalkylenepolyamines are understood to be compounds which containoxyalkylene groups as well as at least two amino groups, preferably atleast two primary amino groups. The polyoxyalkylenepolyamines shouldhave a number average molecular weight of about 137 to 3600, preferably400 to 3000, particularly preferably 800 to 2500. In addition, thepolyoxyalkylenepolyamines should have an amine equivalent weight ofabout 69 to about 1800, preferably 200 to 1500, particularly preferably400 to 1250.

The polyoxyalkylenepolyamines used preferably have a chemical structureaccording to the general formula (I) ##STR1## in which R denotes H or analkyl radical of 1 to 6 carbon atoms, preferably --CH₃

n denotes 5-60, preferably 20-40

The polyoxyalkylenepolyamines which possess a chemical structureaccording to the general formula (I), are disclosed in U.S. Pat. No.3,236,895, column 2, lines 40-72. The methods for the preparation ofthese polyoxyalkylenepolyamines are disclosed in columns 4 to 9 of U.S.Pat. No. 3,236,895 in patent examples 4, 5, 6 and 8 to 12.

It is also possible to use polyoxyalkylenepolyamines which containdifferent oxyalkylene groups, for example polyoxyalkylenepolyamineswhich possess a chemical structure according to the general formula(II):

    H.sub.2 N--CH(CH.sub.3)--CH.sub.2 --(O--CH(CH.sub.3)--CH.sub.2).sub.n --(O--CH.sub.2 --CH.sub.2 --)m--O--CH.sub.2 --CH(CH.sub.3)--NH.sub.2(II)

in which

n+m denotes 5 to 60, preferably 20 to 40

m denotes 1 to 59, preferably 5 to 30

n denotes 1 to 59, preferably 5 to 30

It is also possible to use polyoxyalkylenepolyamine derivatives whichare obtainable by the reaction of the polyoxyalkylenepolyaminesdescribed in U.S. Pat. No. 3,236,895, column 2, lines 40-72, withacrylonitrile, with a subsegment hydrogenation of the reaction product.These derivatives possess a chemical structure according to the generalstructural formula (III):

    H.sub.2 N--(--CH.sub.2 --).sub.3 --NH--CHR--CH.sub.2 --O--(--CHR--CH.sub.2 --O--).sub.n --CH.sub.2 --CHR--NH--(--CH.sub.2 --).sub.3 --NH.sub.2(III)

in which

R denotes H or an alkyl radical of 1 to 6 carbon atoms, preferably --CH₃

n denotes 5 to 60, preferably 20 to 40

The electrocoating baths used according to the invention may of coursealso contain a mixture of several polyoxyalkylenepolyamines of differentchemical structures.

The polyoxyalkylenepolyamines or mixtures of polyoxyalkylenepolyaminesmay be incorporated in the electrocoating baths at any time during thepreparation of the electrocoating baths as well as after these arefinished. The polyoxyalkylenepolyamines or the polyoxyalkylenepolyaminemixtures are preferably added either to the aqueous dispersion or to aprecursor of the aqueous dispersion which contains the cationicamine-modified epoxy resin described in the preamble of patent claim 1and, optionally, crosslinking agents and other customary additives (cf.p. 5, lines 1 to 5), or to the pigment paste or a precursor of thepigment paste (cf. p. 5, lines 8 to 19). The polyoxyalkylenepolyaminemolecules are very probably protonized by the acid contained in theaqueous dispersion or the pigment paste.

However, it is also possible to add thecorrespondingpolyoxyalkylenepolyamine or polyoxyalkylenepolyaminemixture in protonized form to the aqueous dispersion under discussion orto a precursor of this dispersion or to the pigment paste or to aprecursor of the pigment paste. The protonized polyoxyalkylenepolyamineor polyoxyalkylenepolyamine mixture may be obtained by asimple additionof a Bronsted acid to the correspondingpolyoxyalkylenepolyamine orpolyoxyalkylenepolyamine mixture. The total amount of Bronsted acidcontained in the finished electrocoating bath should be chosen such thatthe pH of the electrocoating bath is between 4 and 8, preferably between5 and 7.5.

It is an essential part of the invention that the amount ofpolyoxyalkylenepolyamine or polyoxyalkylenepolyamine mixture containedin the electrocoating baths used according to the invention is at least7.5% by weight, the percentage by weight referring to the total amountof binders contained in the electrocoating bath. In other words thismeans that the electrocoating baths used according to the invention mustcontain at Least 7.5 parts by weight of polyoxyalkylenepolyamine orpolyoxyakylenepolyamine mixture per 100 parts by weightof binder. Ifelectrocoating baths of a lower content ofpolyoxy-alkylenepolyamine orpolyoxyalkylenepolyamine mixture are employed, paint films are obtainedwith a considerably greater number of, and/or considerably more stronglypronounced, surface defects than if electrocoating baths according tothe invention are used.

The upper limits of the amounts of polyoxyalkylenepolyamine orpolyoxyalkylenepolyamine mixture contained in the electrocoating bathsused according to the invention are determined by the plasticizingeffect of the added polyoxyalkylenepolyamine or polyoxyalkylenepolyaminemixture and generally is from 20 to 40% by weight, the percentage byweight referring to the total amount of binders contained in theelectrocoating bath.

The electrocoating baths used according to the invention preferablycontain 8 to 18, particularly preferably 10 to 15% by weight ofpolyoxyalkylenepolyamine or polyoxyalkylenepolyamine mixture, thepercentage by weight referring to the total amount of binders containedin the electrocoating bath.

The total amount of binders contained in the electrocoating bath isobtained by adding the amount of cationic amine-modified epoxy resin ofthe type described in the preamble of patent claim 1 and contained inthe electrocoating bath, the amount of crosslinking agents optionallycontained in the electrocoating bath, the amount of ground resincontained in the electrocoating bath and the amount of resins whichcrosslink under the conditions of baking and which may or may not beadditionally present in the electrocoating bath.

The solids content of the electrocoating bath used according to theinvention is preferably 7 to 35 parts by weight, particularly preferably12 to 25 parts by weight.

The electrocoating bath is brought into contact with an electricallyconducting anode and with an electrically conducting substrate connectedas cathode. When electric current passes between the anode and thecathode, a strongly adhering paint film is deposited on the cathode.

The temperature of the electrocoating bath should be between 15° to 35°C., preferably between 20° to 30° C.

The applied voltage may fluctuate within a considerable range and maybe, for example, between 2 and 1000 volt.

Typical operating voltages are, however, between 50 and 500 volt.Current density is usually between about 10 and 100 ampere/m². Currentdensity tends to drop in the course of the deposition.

After deposition the coated object is rinsed and is ready for baking.

The deposited paint films are generally baked at temperatures from 130°to 200° C. over a period from 10 to 60 minutes, preferably at 150° to180° C. over a period from 15 to 30 minutes.

The process according to the invention may be employed for coating anyelectrically conducting substrate, but particularly for coating metalssuch as steel, aluminum, copper and the like.

The invention is explained in greater detail in the examples below. Allparts and percentages are parts and percentages by weight unlessexpressly stated otherwise.

1. Preparation of aqueous dispersions containing the cationicamine-modified epoxy resin according to the preamble of patent claim 1and a crosslinking agent

1.1 Preparation of the amine-modified epoxy resin

1780 g of Epikote 1001¹), 280 g of dodecylphenol and 105 g of xylene areplaced in a reaction vessel and melted at 120° C. in an atmosphere ofnitrogen. Traces of water are then removed by distillation under aslight vacuum. 3 g of N,N'-dimethylbenzylamine are then added, thereaction mixture is heated to 130° C. and this temperature is kept forabout 3 h, until the epoxide equivalent weight (EEW) has risen to 1162.The reaction mixture is then cooled and 131 g of hexylglycol, 131 g ofdiethanolamine and 241 g of xylene are added in rapid succession. Thisproduces a slight temperature rise. The reaction mixture is then cooledto 90° C. and is further diluted by the addition of 183 g of butylglycoland 293 g of isobutanol. When the temperature has dropped to 70° C., 41g of N,N'-dimethylaminopropylamine are added, this temperature is keptfor 3 h and the mixture is discharged.

The resin has a solids content of 70.2 % and a base content of 0.97milliequivalents/gram.

1.2 Preparation of a crosslinking agent

1.129 g of toluylene diisocyanate (commercial mixture of the 2,4- and2,6-isomers) and 490 g of methyl isobutyl ketone are placed in areaction vessel in an atmosphere of nitrogen. 0.6 g of dibutylzincdilaurate is then added in small portions with 290 g oftrimethylolpropane with stirring at such a rate that with externalcooling the internal temperature does not exceed 50° C. (duration about2 h). Stirring is continued with cooling until the NCO equivalent hasreached the value of 215. 675 g of ethylene glycol monopropyl ether arethen added dropwise at such a rate that the internal temperature doesnot exceed 100° C. The temperature is maintained at 100° C. for afurther 1 hour, the reaction mixture is diluted with 362 g of methylisobutyl ketone and 10 g of n-butanol and is then discharged after abrief cooling period. The resin has a solids content of 71.8 % (1 h at130° C.) and a viscosity of 1.5 dPas (50 % solution in methyl isobutylketone, measured in a plate-cone viscometer).

1.3 Preparation of the aqueous dispersion

1.3.1 Dispersion (I)

915 g of the resin from procedure 1.1, 493 g of the crosslinking agentfrom procedure 1.2 and 134 g of a polyoxypropylenediamine having theformula

    H2N--CH(CH.sub.3)--CH.sub.2 --(--OCH.sub.2 --CH(CH.sub.3))--NH.sub.2

denotes 33.1 (Jeffamine®D 2000, commercial product from Texaco ChemicalCompany) are mixed at room temperature and stirred. As soon as thesolution has become homogeneous, 2.2 g of an antifoam solution¹) and 22g of glacial acetic acid are added with stirring, followed by 674 g ofdeionized water added in 6 portions. The reaction mixture is thendiluted with a further 960 g of deionized water added in small portions.

The resultant aqueous dispersion is freed from low-boiling solvents by avacuum distillation and subsequently diluted with deionized water to asolids content of 33%.

1.3.2 Dispersion (II)

The procedure 1.3.1 is followed except that 67 g ofpolyoxyalkylenepolyamines are used instead of 134 g ofpolyoxyalkylenepolyamines.

1.3.3 Dispersion (III)

The procedure of 1.3.1 is followed, except that nopolyoxyalkylenepolyamine is added. In order to adjust the solids contentto 33%, less deionized water is added after the vacuum distillation.

2. Preparation of a pigment paste

2.1 Preparation of a ground resin in accordance with DE-OS 3,422,457

640 parts of a diglycidyl ether based on bisphenol A and epichlorohydrinof an epoxide equivalent weight of 485 and 160 parts of the same type ofdiglycidyl ether of an epoxide equivalent weight of 189 are mixed at100° C. 452 parts of hexamethylenediamine are placed in a second vessel,heated to 100° C. and treated with 720 parts of the above hot epoxyresin mixture in the course of one hour with slight cooling in order tokeep the temperature at 100° C. After a further 30 minutes the excesshexamethylendiamine is distilled off at elevated temperature and reducedpressure, a final temperature of 205° C. and final pressure of 30 mbarbeing reached. 57.6 parts of stearic acid, 172.7 parts of dimeric fattyacid and 115 parts of xylene are then added. The water formed is thenremoved by azeotropic distillation at 175 to 180° C. over 90 minutes. 58parts of butylglycol and 322 parts of isobutanol are then added added.The product has a solids content of 70% and a viscosity of 2240 mPas,measured at 75° C. in a plate-cone viscometer.

2.2 Preparation of the pigment paste

586 parts of the ground resin are intensively mixed with 1162 parts ofdeionized water and 22 parts of glacial acetic acid. 880 parts of TiO₂,250 parts of an extender based on aluminum silicate, 53 parts of leadsilicate and 10 parts of carbon black are then added. This mixture iscomminuted in a grinding aggregate to a Hegman fineness of less than 12μm. Deionized water is then added in order to attain the desired pasteconsistency.

3. Preparation of the electrocoating baths and depositions according tothe invention

700 parts of the pigment paste from procedure 2.2 are added to 2200parts by weight of the dispersion from procedure 1.3, and the solidscontent of the bath is adjusted to 20% by weight with deionized water.The paint film deposition is carried out during 2 minutes at 300 V ontoa phosphated steel panel. The temperature of the bath is 27° C. Thefilms are baked at 165° C. for 20 minutes.

Electrocoating bath 1: Dispersion from procedure 1.3.1 with paste fromprocedure 2.2

Polyoxypropylenediamine content (based on the total amount of binder):11.9% by weight

Electrocoating bath 2: Dispersion from procedure 1.3.2 with paste fromprocedure 2.2

Polyoxypropylenediamine content (based on the total amount of binders):6.0% by weight

Electrocoating bath 3: Dispersion from procedure 1.3.3 with paste fromprocedure 2.2 Polyoxypropylenediamine content: 0% by weight

    ______________________________________                                        Deposition results                                                            Electroplating bath                                                                          1          2      3                                            ______________________________________                                        Film thickness (μm)                                                                       27         20     16                                           Flow-out.sup.1)                                                                              1.5        2.5    1.5                                          Craters/dm.sup.2                                                                             1          20     10                                           ______________________________________                                    

These films were then overcoated by a commercial aqueous filler and awhite alkyd topcoat and submitted to a condensed water-constant climatetest for 240 h. The adhesion of the films was subsequently tested by thecross-hatch and Tesa pull-off tests.

    ______________________________________                                        Electrocoating bath                                                                          1           2     3                                            ______________________________________                                        Adhesion.sup.1)                                                                              0.5         0.5   0.5                                          ______________________________________                                         .sup.1) Rating 0 to 5 (good  poor)                                       

We claim:
 1. A process for coating electrically conducting substrates,comprising the steps of:(1) immersing the substrate in an aqueouselectrocoating bath which contains a cationic amine-modified epoxy resinobtained by reacting(A) a diepoxy compound of an epoxide equivalentweight below 2000, (B) a compound monofunctionally reactive towardepoxide groups and containing an alcoholic OH group, a phenolic OH groupor an SH group, and (C) an amine, the components (A) and (B) being usedin molar ratio of 10:1 to 1:1 and the positive charges being introducedby protonization of the reaction product, by use of an amine salt ascomponent (C) or a combination thereof; (2) connecting the substrate toa current source to act as a cathode, (3) depositing a film on thesubstrate by the action of direct current, (4) removing the substratefrom the electrocoating bath and (5) baking the deposited paint film,wherein the electrocoating bath further contains at least 7.5% by weightof a polyoxyalkylenepolyamine, wherein the polyoxyalkylenepolyamine isselected from the group consisting of chemical structures having theformula:

    H.sub.2 N--CHR.sub.1 --CH.sub.2 --O--).sub.n --CH.sub.2 --CHR.sub.1 --NH.sub.2

in which R₁ denotes H or an alkyl radical or 1 to 6 carbon atoms, and ndenotes 5-60,

    H.sub.2 N--CH(CH.sub.3)--CH.sub.2 --(O--CH(CH.sub.3)--CH.sub.2).sub.m --(O--CH.sub.2 --CH.sub.2 --).sub.o --O--CH.sub.2 --CH(CH.sub.3)--NH.sub.2

in which m+o denotes 5 to 60, m denotes 1 to 59, and o denotes 1 to 59,and

    H.sub.2 N--(CH.sub.2).sub.3 --NH--CHR.sub.2 --CH.sub.2 --O--(--CHR.sub.2 --CH.sub.2 --O--).sub.p --CH.sub.2 --CHR.sub.2 --NH--(CH.sub.2).sub.3 --NH.sub.2

in which R₂ denotes H or an alkyl radical of 1 to 6 carbon atoms, and pdenotes 5 to 60, the percentage by weight referring to the total amountof binder contained in the electrocoating bath.
 2. The process asclaimed in claim 1, wherein the electrocoating bath further contains ablocked polyisocyanate as crosslinking agent.
 3. The process as claimedin claims 1 or 2, wherein the polyoxyalkylenepolyamine is

    H.sub.2 N--CHR--CH.sub.2 --O--(--CHR--CH.sub.2 --O--).sub.n --CH.sub.2 --CHR--NH.sub.2

in which R denotes H or an alkyl radical of 1 to 6 carbon atoms, ndenotes 5 to
 60. 4. An aqueous electrocoating bath comprising: acationic amine-modified epoxy resin obtainable by reacting(A) a diepoxycompound of an epoxide equivalent weight below 2000, (B) a compoundmonofunctionally reactive toward epoxide groups and containing analcoholic OH group, a phenolic OH group or an SH group, and (C) anamine, the components (A) and (B) being used in a molar ratio of 10:1 to1:1 and positive charges being introduced by protonization of thereaction product or use of an amine salt as the component (C), whereinthe electrocoating contains at least 7.5% by weight of apolyoxyalkylenepolyamine, wherein the polyoxyalkylenepolyamine isselected from the group consisting of chemical structures having theformula:

    H.sub.2 N---CHR.sub.1 --CH.sub.2 --O--(--CHR.sub.1 --CH.sub.2 --O--).sub.n --CH.sub.2 --CHR.sub.1 --NH.sub.2

in which R₁ denotes H or an alkyl radical of 1 to 6 carbon atoms, and ndenotes 5-60,

    H.sub.2 N--CH(CH.sub.3)--CH.sub.2 --(O--CH(CH.sub.3)--CH.sub.2).sub.m --(O--CH.sub.2 --CH.sub.2 --).sub.o --O--CH.sub.2 --CH(CH.sub.3)--NH.sub.2

in which m+o denotes 5 to 60, m denotes 1 to 59, and o denotes 1 to 59,and

    H.sub.2 N--(CH.sub.2).sub.3 --NH--CHR.sub.2 --CH.sub.2 --O--(--CHR.sub.2 --CH.sub.2 --O--).sub.p --CH.sub.2 --CHR.sub.2 --NH--(CH.sub.2).sub.3 --NH.sub.2

in which R₂ denotes H or an alkyl radical of 1 to 6 carbon atoms, and pdenotes 5 to 60, the percentage by weight referring to the total amountof binder contained in the electrocoating bath.
 5. An electrocoatingbath as claimed in claim 4, further comprising a crosslinking agent, theagent being a blocked polyisocyanate.
 6. An electrocoating bath asdescribed in claim 4 or 5 wherein the polyoxyalkylenepolyamine is

    H2N--CHR--CH.sub.2 --O--(CHR--CH.sub.2 --O--).sub.n --CH.sub.2 --CHR--NH.sub.2

in which R denotes H or an alkyl radical of 1 to 6 carbon atoms, and ndenotes 5 to
 60. 7. The process of claim 1 wherein the components (A)and (B) are used in a molar ratio of 4:1 to 1.5:1.
 8. The process ofclaim 3 wherein R is --CH₃.
 9. The process of claim 3 wherein n denotes20 to
 40. 10. The electrocoating bath of claim 6 wherein R is --CH₃. 11.The electrocoating bath of claim 6 wherein n denotes 20 to
 40. 12. Aprocess for electrocoating a substrate in an electrocoating bath,comprising cathodically electrodepositing onto said substrate a cationicamine-modified epoxy resin comprised of the reaction product of (A) adiepoxy compound having an epoxide equivalent weight below 2000; (B) acompound monofunctionally reactive toward the epoxide group of (A) andcontaining an alcoholic --OH group, a phenolic --OH group or an --SHgroup and (C) an amine,the components (A) and (B) being used in a molarratio of 10:1 to f1:1, said reaction product being positively charged byprotonization of the reaction product, said electrocoating bath furthercontaining at least 7.5% by weight of a polyoxyalkylenepolyamine,wherein the polyoxyalkylenepolyamine is selected from the groupconsisting of chemical structures having the formula:

    H.sub.2 N--CHR.sub.1 --CH.sub.2 --O--(--CHR.sub.1 --CH.sub.2 --O--).sub.n --CH.sub.2 --CH.sub.2 --CHR.sub.1 --NH.sub.2

in which R₁ denotes H or an alkyl radical of 1 to 6 carbon atoms, and ndenotes 5-60,

    H.sub.2 N--CH(CH.sub.3)--CH.sub.2 --(O--CH(CH.sub.3)--CH.sub.2).sub.m --(O--CH.sub.2 --CH.sub.2 --).sub.o --O--CH.sub.2 --CH(CH.sub.3)--NH.sub.2

in which m+o denotes 5 to 60, m denotes 1 to 59, and o denotes 1 to 59,and

    H.sub.2 N--(CH.sub.2).sub.3 --NH--CHR.sub.2 --CH.sub.2 --O--(--CHR.sub.2 --CH.sub.2 --O--).sub.p --CH.sub.2 --CHR.sub.2 --NH--(CH.sub.2).sub.3 --NH.sub.2

in which R₂ denotes H or an alkyl radical of 1 to 6 carbon atoms, and pdenotes 5 to 60, based upon the total weight of the binders contained inthe bath.
 13. A process for electrocoating a substrate in anelectrocoating bath comprising cathodically electrodepositing onto saidsubstrate a cationic amine-modified epoxy resin comprised of thereaction product (A) a diepoxy compound having an epoxide equivalentweight below 2000; (B) a compound monofunctionally reactive toward theepoxides of (A) and containing an alcoholic --OH group, a phenolic --OHgroup or an --SH group and (C) an amine, the components (A) and (B)being used in a molar ratio of 10:1 to 1:1, and the positively chargesbeing introduced by protonization of the reaction product or use of anamine salt as the component (C) said bath further containing at least7.5% by weight of a polyoxyalkylenepolyamine, wherein thepolyoxyalkylenepolyamine is selected from the group consisting ofchemical structures having the formula:

    H.sub.2 N--CHR.sub.1 --CH.sub.2 --O--(--CHR.sub.1 --CH.sub.2 --O--).sub.n --CH.sub.2 --CHR.sub.1 --NH.sub.2

in which R₁ denotes H or an alkyl radical of 1 to 6 carbon atoms, and ndenotes 5-60,

    H.sub.2 M--CH(CH.sub.3)--CH.sub.2 --(O--CH(CH.sub.3)--CH.sub.2).sub.m --(O--CH.sub.2 --CH.sub.2 --).sub.o --O--CH.sub.2 --CH(CH.sub.3)--NH.sub.2

in which m+o denotes 5 to 60, m denotes 1 to 59, and o denotes 1 to 59,and

    H.sub.2 N--(CH.sub.2).sub.3 --NH--CHR.sub.2 --CH.sub.2 --O--(--CHR.sub.2 --CH.sub.2 --O--).sub.p --CH.sub.2 --CHR.sub.2 --NH--(CH.sub.2).sub.3 --NH.sub.2

in which R₂ denotes H or an alkyl radical of 1 to 6 carbon atoms, and pdenotes 5 to 60, based upon the total weight of the binders contained inthe bath.